External drug pumps are typically used to deliver to patients substances which contain large molecules which cannot be digested when administered orally, such as insulin. Typically, the pump is adhered to the abdomen of the patient and delivers the substance to the patient via a cannula or needle that is inserted into the patient's skin.
PCT Patent Application PCT/IL2008/001312 (published as WO 2009/044401) to Gross and Cabiri (as well as U.S. patent applications Ser. Nos. 12/244666 and 12/244668, the disclosures of which are incorporated herein by reference), describes an external drug pump. In this pump, a cartridge is provided that contains a substance to be administered to a subject. (The terms “drug” and “substance” are used interchangeably throughout the specification and claims, and encompass any material administered to a subject. The term “cartridge” throughout the specification and claims encompasses any container for a drug, such as but not limited to, a cartridge, vial, syringe, bottle, ampoule and many more, and is not limited to any size or shape.)
The cartridge is sealed by a stopper, and has first and second threaded elements (e.g., a screw and a nut) that are threadedly coupled to each other. The distal end of the second threaded element defines a coupling portion that couples the second threaded element to the stopper. The first threaded element is rotatable with respect to the cartridge, and is linearly immobile with respect to the cartridge during rotation of the first threaded element. The first threaded element, rotated by a motor, is configured to linearly advance the stopper and at least the distal end of the second threaded element toward the distal end of the cartridge, without substantially rotating the second threaded element and the stopper.
The following is provided to facilitate understanding of the above described assembly.
Reference is made to FIG. 1, which illustrates the relevant elements of the prior art cartridge assembly of WO 2009/044401 (based on FIG. 4 of that application), wherein a cartridge 22 is inserted into a housing base.
The distal end of cartridge 22 is inserted into a cartridge piercing mechanism 44, which pierces a seal at the distal end of cartridge 22 having a stopper 24 therein (the stopper being an example of a plunger, piston or pushing device; the stopper will also be referred to as a plunger). Cartridge 22 is then lowered into the housing base. Typically, opposing resilient arms 70 support the cartridge upon the housing base. As cartridge 22 is lowered into the housing base, a first cog 52 engages a second cog 54. (First cog 52 is rotated by the motor, not shown here.) In some applications, before insertion of cartridge 22 into the housing, first threaded element 26 protrudes a distance h from the proximal end of the cartridge. The proximal end of the first threaded element (or of second cog 54) comprises a rounded portion 74. Portion 34 of the housing base comprises an angled face 76. As rounded portion 74 slides past the angled face, the first threaded element is pushed the distance h inside the cartridge. As a result, the first and second threaded elements 26 and 28 and the stopper 24 are displaced towards the distal end of the cartridge 22. During operation, the motor (not shown) turns cog 52, which turns cog 54. This linearly advances stopper 24 towards the distal end of the cartridge 22, thereby administering the substance from cartridge 22.
Reference is now made to FIG. 2, which illustrates cartridge 22 with plunger 24 in an initial position before cartridge 22 has been filled. The cartridge is either pre-filled by the manufacturer or filled by the user, such as with a hypodermic needle 15 inserted through a septum 17 in cartridge 22 (FIG. 3) or through a septum 19 in the housing of the drug pump (FIG. 4). As shown in FIG. 3, the plunger 24 moves linearly in the cartridge 22 as the cartridge is filled. Because no two cartridges will be filled with exactly the same amount of substance, such as due to tolerances, different injected volumes, different diameters or other dimensions, different plunger shapes, air bubbles or other factors, there is unfortunately no definitive position of the plunger 24 after cartridge 22 has been filled. This can cause a problem because it is possible that the overall length of the geared and threaded mechanism (which is predefined) that interfaces with plunger 24 may not meet the filled position of plunger 24 (which is unknown), thereby causing a problem to properly push against plunger 24 in order to administer the substance.